1. Field of the Invention
This invention relates to novel microporous membranes, and is partially concerned with expanded polytetrafluoroethylene (PTFE), or expanded interpenetrating polymer networks (IPN) of PTFE and silicone.
2. Description of the Prior Art
In U.S. Pat. No. 3,315,020, there is disclosed a process for preparing sheet-like articles of PTFE which, in the unsintered state, have high elongation and strength in all directions in the major plane of the sheet. In this process, a relatively large cylinder of compacted dispersion grade PTFE particles is passed through a die having two orifices in series. The first orifice is a round, square or rectangular orifice and the second orifice is a long slit-shaped orifice. The extrusion of particulate PTFE is aided by pre-blending with an organic fluid lubricant such as, for example, kerosene, VM&P naphtha or Isobars. The lubricant aids plastic flow of the PTFE particles which coalesce into a cohesive paste under pressure. The extrusion process thereby produces a continuous ribbon of PTFE partially saturated with volatile organic liquid.
The PTFE extrudate ribbon is usually calendered at room temperature while still partially saturated. Calendering involves compression of the ribbon between nip rolls spaced apart by a predetermined distance. Such distance is substantially less than the thickness of the entering extrudate ribbon. Since the entering extrudate ribbon contains lubricant and cold flow is easily effected, elongation of the article in the machine direction occurs by plastic flow in proportion to the difference between entering and exiting thickness at the calender. Normally, the exiting extrudate ribbon is then heated by means of hot rollers to a temperature at which the organic lubricant is volatilized in a reasonable period of time to produce a dried extrudate ribbon.
In the manufacture of PTFE pipe-thread sealant, further orientation of the dried extrudate ribbon is accomplished by linear stretching using differential speed rollers. The amount of stretching is determined by the ratio of the speed of the second roll to the first, i.e., a ratio of 1.75:1 represents an elongation of 75 percent.
In the production of microporous PTFE membranes, linearly oriented extrudate ribbon is stretched in the transverse direction by use of equipment such as a tenter frame or the like. A tenter frame is an apparatus which consists of two gripping conveyors that grab the ribbon by its edges. The conveyors move farther away from one another as they move forward into a tunnel oven. At the end of the oven the grips release. The extrudate ribbon is thereby stretched in the transverse direction in a continuous fashion at elevated temperatures, and may be easily taken-up into rolls.
When the organic lubricant is removed and PTFE extrudate ribbon is stretched, the material does not become thinner as would an ordinary plastic. Instead, PTFE has the unusual ability to undergo an internal fracture process referred to as fibrillation. The PTFE is initially a cohesive bulk. At first application of tensile stress, the bulk breaks apart into clumps of material called nodes. These nodes are connected to each other by fibrils, which are essentially strands of oriented PTFE molecules that are pulled from the nodes. The material thereby becomes microporous, with the pore architecture being determined by the empty space defined by the nodes and fibrils.
In this fashion, the amount of stretching and the ratio of linear to transverse stretching control the pore architecture. As the degree of stretching is increased, the nodes break into smaller structures and increasing amounts of fibrils are created. Thus porosity is increased, but bulk dimensions may not be significantly changed. In membrane manufacture, a sheet may be stretched several thousand percent but only lose half of its thickness, resulting in a net increase in bulk volume. Hence, the process of stretching PTFE into fibrillated structures is referred to as expansion. This effect manifests itself in that some forms of expanded PTFE sheeting can have a negative Poisson ratio in the direction perpendicular to the major planes of the sheet, (Cherfas, 1990).
A collateral effect of the fibrillation process is that increased stretching does not necessarily create larger pores, but may actually decrease the mean pore size of the final product. A relatively small amount of larger pores gives way to a larger amount of small pores. With biaxially expanded PTFE produced from the tenter process, high amounts of linear stretching typically produce a relatively small pore size in the final product. However, with extreme amounts of stretching in either direction, the pore size of such membranes eventually increases as the mass of PTFE nodes is depleted. At this point the bulk material behaves more plastically and further stretching simply elongates the fibrils and increases the distance between nodes until macroscopic holes are produced.
PTFE dispersion grade resin is approximately 95% crystalline, and has a very high melt viscosity. When heated to temperatures above 327.degree. C., the PTFE crystallites deform into an amorphous polymer. When cooled below 327.degree. C., some of the liquid structure remains; thereby increasing the amorphous content of melted PTFE in comparison to virgin PTFE. Such heating and subsequent cooling of the polymer is called sintering. The sintering of shaped microporous PTFE structures produces a thermosetting effect upon the polymer, thus enabling shape retention and thermal stability. Microporous PTFE membranes are typically sintered by creating a high temperature zone in the final stages of the tenter frame apparatus.
In the prior art process of producing PTFE extrudate, difficulties arise in attempts to produce uniform extrudate having satisfactory molecular orientation to facilitate adequate transverse stretching. The molecular orientation of the extrudate ribbon is highly influenced by the rheological properties of the polymer formulation, the pressure applied to the system, and the design of the die. Various grades of PTFE resin provide distinct rheological conditions during extrusion due to different physical and molecular characteristics, therefore different grades of resin produce different degrees of molecular orientation even under the same extrusion conditions. PTFE extrusion systems are complex and difficult to control. Examples of these difficulties are found in U.S. Pat. No. 4,187,390.
In U.S. Pat. No. 4,945,125, there is disclosed a process of producing fibrillated IPNs of PTFE and polyorganosiloxanes. The IPNs are formed by either a sequential method or a simultaneous method. The simultaneous method involves the pre-blending of polysiloxane compositions with the organic lubricant used for extrusion. The siloxane/lubricant blend is then dispersed into the particulate PTFE resin, which is then extruded under similar conditions as those of the prior art. The extrusion and expansion of such IPN products was found to be facilitated by the presence of the siloxane, which apparently acts as an internal lubricant for molecular orientation, and plays a roll in node/fibril formation after volatilization of the organic lubricant.
When skillfully produced, IPN products have been shown to possess increased strength and uniformity in comparison to conventional PTFE products.
In the prior art of producing microporous PTFE membranes or microporous IPN membranes, the pore architecture is homogeneous across the thickness of the sheet. This is due to the fact that (a) the polymer composition, and therefore the rheological extrusion conditions, are homogeneous across the thickness of the extrudate sheet, (b) the orientation caused by the calendering process is applied uniformly throughout the extrudate ribbon, (c) expansion of the extrudate in either direction is applied uniformly across the thickness of the sheet, and (d) sintering affects the entire microstructure of the expanded film.
Those skilled in the art will be familiar with several methods of laminating membranes with different thickness and pore structures together, such as adhesive lamination or thermobonding. With adhesive bonding, two or more layers of microporous membranes are glued together to give the desired pore size gradient. Such adhesives have the detrimental effect of blocking the flow of fluid or gas in proportion to the area covered by the adhesive. Such adhesive systems are also prone to delamination under demanding conditions. Thermobonding is a process which relies on heat and pressure to effect lamination. This process is particularly problematic for PTFE-like polymers which have a very high melting temperature and/or viscosity. With sufficient pressure and temperature, however, microporous PTFE or PTFE/silicone IPN membranes can be laminated together. However, such conditions can adversely effect the microstructure of the membranes by causing deformation. Thermobonded membranes are also prone to delamination because the bonding of the two layers tends to be superficial since the degree of bonding is dependent on the number of contact points between nodes of each membrane and the extent to which molten mass flow is achieved. The high melt viscosity of PTFE limits the effectiveness of this technique.